This invention relates in general to optical fibers and in particular to hermetic coatings for optical fibers. It is well known that bare uncoated fibers are susceptible to abrasion resulting in surface scratches. These scratches can produce a significant loss of light through the sides of the fiber and can also result in sudden failure through breakage of the fiber. Optical fibers are susceptible to breakage not only because they are formed from relatively brittle materials, but in addition the fibers typically have very small diameters and are subjected to an assortment of stresses such as bending stresses and tensile stresses. These stresses tend to open cracks or scratches thereby focussing the strain onto the chemical bonds at the tip of the crack. This results in growth of the scratch or crack eventually resulting in sudden failure of the fiber through breakage. It has therefore become standard to protect an optical fiber from abrasions by coating it with an abrasion resistant substance such as silicone. This abrasion resistant coating is typically applied on-line as the optical fiber is drawn so that surface abrasion is avoided during the pulling process.
It has more recently been recognized that optical fibers are also susceptible to corrosion. Various chemicals, including water, can react with a fiber damaging its optional properties and weakening its mechanical strength and static fatigue resistance. Microcracks in a fiber surface present regions susceptible to chemical attack, especially when the fiber is under stress. Fiber stress tends to open a crack, thereby focusing the strain onto the chemical bonds at the tip of the crack. These strained bonds are more easily chemically attacked thereby enabling corrosion to extend such microcracks. Growth of microcracks weakens the strength of a fiber producing static fatigue or sudden failure.
The effect of stress corrosion on the time to failure t.sub.s of an optical fiber under static stress .sigma..sub.s at ambient temperature and humidity results in a linear variation of log t.sub.s with log .sigma..sub.s : EQU log t.sub.s =-n log .sigma..sub.s +log B+(n-2) log S.sub.i
where B is a constant for a given glass and test environment, n is the crack velocity exponent and S.sub.i is the fracture strength in an inert environment. (See, J. E. Ritter, Jr., Fiber and Int. Optics, 1, 387 (1978).
The crack velocity exponent n is generally a useful parameter for evaluating optical fibers. Typically, fibers having a large value of n also have a large value of t.sub.s under typical values of applied stress. In addition, a fiber having a large value of n exhibits a rapid variation of t.sub.s with .sigma..sub.s. This enables a fiber having a large value of n to be tested for a reasonable test period (e.g. a few days) at a stress only slightly above that to which it will be subjected under use. If the fiber doesn't break during this test period then, because, for large n, log t.sub.s is a rapidly decreasing function of log .sigma..sub.s it is certain to last for a long time under typical use conditions. Therefore, such fibers can be easily tested for purposes of quality control.
There presently exist a variety of coatings which protect a fiber from abrasion but not from corrosion. Because such coatings do not protect a fiber from corrosion, many prior art methods attempt to reduce microcrack degradation by employing expensive techniques to reduce the number of microcracks. Additionally, the use of fibers is often restricted to low stress applications. Another approach is to apply a metallic coating to a fiber to prevent water from reaching the fiber. It has been suggested to apply a metal seal of molten tin or aluminum which form a hermetic coating when cooled. However, metals tend to form polycrystalline solids which can themselves be rapidly corroded via greatly enhanced grain boundary diffusion. The relatively open structure of the grain boundaries provide an easy path for migrating ions to reach the SiO.sub.2 surface and nucleate and/or propagate cracks. Metal coats also provide an often undesirable electrical path along a fiber. In addition, many metals react with SiO.sub.2 to form metal oxides. These abrasive particles of metal oxides may roughen the SiO.sub.2 surface and act as stress raisers to provide easy nucleation sites for potential cracks.
Several non-metallic coatings have been utilized to produce a hermetic seal on optical fibers. For example, silicon nitride (See U.S. Pat. No. 4,028,080 entitled "Method of Treating Optical Waveguide Fibers", issued to DiVita et al on June 7, 1977) and carbon (See U.S. Pat. No. 4,183,621 entitled "Water Resistant High Strength Fibers" issued to Kao et al. on Jan. 15, 1980) have been utilized to hermetically seal optical fibers.
Unfortunately, coatings which are suitable for use at ambient conditions are not necessarily suitable for use at more extreme temperatures and/or pressures. The increasing use of optical fibers is resulting in their use under conditions not previously encountered. For example, if optical fibers are to be used in borehole logging operations (i.e., the accumulation of data from instruments lowered down the borehole of an oil well), then the fibers must function for a useful lifetime under the extreme temperatures (on the order of 200 degrees Centigrade) and pressures (on the order of 20,000 psi) which can be encountered in a typical oil well. For a fiber to be usable for 8 hours a day over a 2 year period, this requires a 4000 hour lifetime under these extreme conditions.
Cables lowered down an oil well typically are subjected to a 3% strain so that the hermetic coating must remain intact under such strains. When conventional glass fibers under 2-3% strain (1.4-2.1-10.sup.9 N/m.sup.2 stress) are immersed in water at the relatively mild conditions of 95 degrees Centigrade and one atmosphere pressure, static fatigue or corrosion cracking limits their useful lifetime to 3-4 days. Since the extreme pressures in an oil well force water through the abrasion resistant coating, it is important that a hermetic coating be found which remains intact for a useful lifetime when immersed in water at 200 degrees Centigrade and 0.14 10.sup.9 N/m.sup.2 pressure.
When optical fibers are immersed in water, the dependence of log t.sub.s on log .sigma..sub.s varies from the linear dependence exhibited in air. In FIG. 2 is shown experimental static fatigue data for a conventional optical fiber immersed in water at assorted temperatures. These data shows that the time to failure for such fibers is not only on the order of one day at 2.1 10.sup.9 N/m.sup.2 stress (i.e. 3% strain), these curves also exhibit a bend in the vicinity of 2.4 10.sup.9 N/m.sup.2 stress. Similar curves also result for optical fibers protected with a silicon nitride coating. At 3% strain, these fibers also break within a few days. Ellipsometry tests of the fiber indicate that the silicon nitride coating significantly decreases in thickness during that period. Silicon nitride has therefore turned out to be unsuitable for use in the conditions encountered in borehole logging. Similarly fibers coated with carbon have also been tested. For a 300 angstrom carbon coating deposited by sputtering, the crack velocity exponent n was determined to be only 30.3 (See M. L. Stein et al., "Ion Plasma Deposition of Carbon-Indium Hermetic Coatings for Optical Fibers", Proc. of CLEO Conference of Laser and Electrooptics, Washington, D.C., June 10-12, 1981). For the case of a 100 angstrom coating deposited on-line by chemical vapor deposition, the value of n was only 8. These tests therefore indicate that carbon is not a suitable hermetic coating for optical fiber immersed in water.
Silicon carbide and related coatings having a range of ratios of silicon, carbon, nitrogen and oxygen have been tested while immersed in water and have proven to have lifetimes which are several orders of magnitude better than that shown for silicon nitride and carbon coatings. The range of ratios enables a coating to be selected which not only remains hermetic over a useful lifetime while immersed in water, but also matches the physical properties (e.g., bulk modulus and thermal coefficient of expansion) of the optical fiber as closely as possible while retaining hermeticity when immersed in water.